A liquid crystal display device is fabricated such that an upper substrate and a lower substrate are attached facing each other, and a liquid crystal layer is disposed between the upper substrate and the lower substrate. A color filter layer transmitting a light of a specific wave length and a common electrode producing an electric field are formed on the upper substrate. Gate and data lines crossing each other, a thin film transistor for a switching element, and a pixel electrode for producing an electric field are formed on the lower substrate. The liquid crystal display device operates by using an optical anisotropy and a polarization property of liquid crystal molecules. The liquid crystal molecules have a molecular arrangement direction, and thus the direction is controlled by supplying an electric field to the liquid crystal molecules. That is, the arrangement direction of the liquid crystal molecules is changed by supplying an electric field to the liquid crystal molecules with by switching the thin film transistor, and thus a transmittance of the light is controlled. The light passing through the liquid crystal molecules passes through the color filter, and thus the liquid crystal display device displays images.
The liquid crystal display device is a non-emissive type display device using a light source. For example, a cold cathode fluorescent lamp (CCFL) is used as a light source, and the lamp is driven by an inverter. The CCFL of the liquid crystal display device can be driven with an alternating current (AC) voltage of approximately 840V.
As shown in FIG. 1, the driving circuit for inverter 100 is partitioned into a rectification/smoothing unit, a direct current/direct current converter (DC/DC converter), and a direct current/alternating current inverter (DC/AC inverter). When an AC 220V is inputted, the rectification/smoothing unit with a bridge rectifier 110 rectifies and smoothes the AC 220V to generate a DC 311V. When a power factor correction (PFC) function by a power saving circuit (not shown) is used in order to improve a power efficiency, the AC 220V may be rectified and smoothed to be a DC 400V. Accordingly, the AC 220V inputted to the rectification/smoothing unit may be rectified and smoothed to the DC 311 V or 400V. Then, the DC 311 V or 400V is changed to a DC 12V to 24V through the DC/DC converter with a switching mode power supply transformer (SMPS transformer) 120, and then inputted to the DC/AC inverter. Then, the DC 12V to 24V are changed to an AC 840V for driving a lamp 140 through the DC/AC inverter with a transformer 130, thereby driving the lamp 140. The transformer 130 is generally a magnetic transformer.
A ground of the driving circuit for inverter 100 is divided into a hot ground and a cold ground by the SMPS transformer 120. In general, the cold ground may be referred to as an earth. The hot ground is a ground for the DC voltage rectified and smoothed from the AC voltage, and may inflict a bodily injury on a person. A voltage difference between the hot ground and the cold ground may be about 100V.
The driving circuit for inverter according to the related art has two transformation stages through the two transformers 120 and 130, and power is consumed through each stage. More particularly, the efficiency through the rectification/smoothing unit is 95%, the efficiency through the DC/DC converter is 90%, and the efficiency through the DC/AC inverter is 76.4%.